The human consequences of climate change also fall into two categories: impacts and responses. The impacts of climate change denote a positive or negative change in a natural or human system caused by its exposure to climate change. A positive impact on an agricultural system, for example, would be increased yields, but a negative impact would be decreased yields. The severity of the impacts is determined by the vulnerability of that system; that is, how badly the system could be hurt by its exposure to climate change. All things being equal, a crop that is little affected by rising temperature is less vulnerable than one that is greatly affected by the same temperature increase. A farmer who has the knowledge or resources to change his farming practices to match the new temperature regime is less vulnerable than a farmer who lacks sufficient knowledge or resources to adjust.
Humans can respond to climate change impacts in two ways. First, they can address the causes of climate change through mitigation. Mitigation involves actions that prevent, limit, delay, or slow the rate of climate change. Mitigation can involve direct interventions in the natural environment, direct interventions in the proximate causes, or indirect interventions through the driving forces. An example of mitigation would be government policies aimed at reducing the number of cars on the road and, consequently, the carbon dioxide emissions from tail pipes. As noted above, upcoming lessons will delve more deeply into the topic of mitigation.
Humans can also reduce vulnerability to climate change impacts through adaptation. Adaptation involves actions taken in response to climate change that enhance compatibility with the new environment by reducing vulnerability and building resilience. For instance, coastal communities can take many actions to address the impacts of sea level rise: build sea walls, increase natural vegetation to reduce erosion, or retreat from the coast by promoting inland development and preventing further coastline development. Lesson 3 focuses on climate impacts and adaptation, with special attention to vulnerability and resilience. Lessons 7 through 10 address climate impacts and adaptation in four critical human systems––water resources, coastal zones, human health, and cities.